The present invention relates to a method and relative system for the early identification of malfunctionings of the drill bit due to wear of the same during the drilling of hydrocarbon wells.
The state of the art whereto the present invention relates is that of explorations of the subsoil aimed at exploiting its geothermal and oil resources.
In the procedures of drilling of a well for the extraction of hydrocarbons the choice of the drill bit of the auger is of fundamental importance for the purpose of efficiency of drilling. This choice is carried out, in general, by taking as reference the performances of drill bits used previously in nearby and/or similar contexts and wells (offset wells) or in lithotypes which are expected to be encountered also in the well in question. More particularly, drill bits of different materials and shapes are chosen, according to the features of mechanical resistance to the compression of the lithotypes foreseen.
In the case of “soft rock formations” (e.g. non-consolidated sands, clays, soft limestone, shale, red beds, etc.), drill bits are used whose cutting surface is provided with long teeth or elements with chisel shape distanced once from the other, which allow high rates of advancement with little weight and a high number of revs. In the case where traversing of “medium rock formations” (calcite, dolomite, medium limestone, etc.) is foreseen, drill bits with teeth that are close one in relation to the other will be chosen, with respect to those of drill bits used for soft rocks. Finally, the possibility is foreseen of encountering “hard rock formations” (e.g. hard sands, hard calcite, mudstones, etc.), the drill bits to be chosen will have teeth that are even shorter and closer together compared to the previous cases and work more by abrasion than by compression. While the shorter inserts, on the one hand, have lower rates of advancements caused by a rather high number of revolutions, and little weight on the drill bit, on the other hand they have a greater resistance to fatigue and can remain in the well for a longer time before wearing out.
In order to choose the most suitable type of drill bit and best optimise the drilling parameters, it would be necessary, therefore, to know exactly the lithology of the area is going to be drilled. The information that is available before digging a well, such as, for example, that derived from adjacent and/or similar contexts such as offset wells can, however, give only an approximate idea of the actual situation of the subsoil.
Situations may, therefore, occur in which the initial forecasts are wrong and the drill bit is incorrectly selected with respect to what the real lithology of the well required. This leads to premature wear of the drill bit, which becomes unusable before having reached the required depths. This wear can go as far as to cause the breakage of the drill bit with the detachment of pieces of the drill bit itself. When the drill bit has to be extracted and replaced, the manoeuvre of extraction and descent of the new drill bit may take from many hours up to a few days, in cases where it is also necessary to recover any fragments of non-negligible dimensions, worn, detached from the worn drill bit. The times required to bring the worn drill bit and its possible fragments back up and the times of descent of the new drill bit are, obviously, dead times, which increase in a non-negligible way the cost of drilling.
Similar situations may occur in the event of malfunctioning of the drill bit due, not to a wrong choice of the same with respect to the lithology, but simply to the consumption of the drill bit through fatigue. The average life of a drill bit is, in fact, in any case limited, even when this is used with the rocks most suitable for its morphology. It is obvious that, also in the case of wear due simply to fatigue, it is important to be able to realise as soon as possible that the drill bit is now impaired, to avoid unnecessarily continuing to drill and to prevent that there are breakages with detachment of fragments of the same drill bit, resulting in a waste of working hours.
The only parameter which, currently, can be used as a possible indicator of the fact that the drill bit is not able to drill adequately, or because its morphology is not suited to the rock formations encountered, or because it is worn due to fatigue, is the rate of penetration. The latter is commonly known in the industry with the expression “rate of penetration” (ROP). The first suspicion that there is an error in the choice of the drill bit or that the drill bit is consumed due to fatigue, occurs, in fact, when there is an abrupt drop in said rate of penetration. A sudden decrease in the rate of penetration, however, may not necessarily be caused by a malfunctioning of the drill bit, but also by any anisotropies of the lithology of the subsoil during drilling. It is possible, in fact, that a temporary drop in the rate of penetration occurs, without this entailing the drill bit being replaced. Bringing the drill bit back to the surface simply because a decrease in the rate of penetration has been observed may be a mistake if this decrease is due to causes different from wear of the drill bit. Vice versa, it may happen that drilling continues, waiting until the rate of penetration is truly reduced to the minimum. In such cases, that is if the drilling is not interrupted in time, in addition to the waste of time and resources due to the fact that drilling continues with low efficiency, the fracture of the drill bit parts may even take place. As already mentioned above, these parts remain at the bottom of the well and should, therefore, be recovered, with a further waste of hours of work. In light of these considerations it can clearly be seen that the single rate of penetration is not enough to be able to take any decisions on interruption of drilling and on the recovery of the drill bit at the surface.
At the state of art, some methods are known, that in order to overcome this limitation, merge the information about the rate of penetration of the auger with other information, such as, the hydrogen concentration in the drilling mud. The patent application CA2223662A1 describes, indeed, a method for the identification of malfunctioning of the drill bit, which, includes the measurement of the drilling depth and of the rate of penetration of the drill bit, the measurement of hydrogen concentration in the drilling mud and the use, as indicator of malfunctioning of the drill bit, of the combination of the increasing of the hydrogen concentration and of the decreasing of the rate of penetration. In the patent application US 2013/008717, are also mentioned the radioisotope concentration and the concentration of nickel, zinc, silver or copper, as indicators of malfunctioning.